Split HookContext and HookCommandContext to prevent deadlocks

HookContext (all events):
- isIdle() - read-only state check
- hasQueuedMessages() - read-only state check
- abort() - fire-and-forget, does not wait

HookCommandContext (slash commands only):
- waitForIdle() - waits for agent to finish
- newSession(options?) - create new session
- branch(entryId) - branch from entry
- navigateTree(targetId, options?) - navigate session tree

Session control methods moved from HookAPI (pi.*) to HookCommandContext (ctx.*)
because they can deadlock when called from event handlers that run inside
the agent loop (tool_call, tool_result, context events).

packages/coding-agent/test/compaction-hooks.test.ts

@@ -80,9 +80,6 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
-			setNewSessionHandler: () => {},
-			setBranchHandler: () => {},
-			setNavigateTreeHandler: () => {},
 		};
 	}
 
@@ -269,9 +266,6 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
-			setNewSessionHandler: () => {},
-			setBranchHandler: () => {},
-			setNavigateTreeHandler: () => {},
 		};
 
 		createSession([throwingHook]);
@@ -319,9 +313,6 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
-			setNewSessionHandler: () => {},
-			setBranchHandler: () => {},
-			setNavigateTreeHandler: () => {},
 		};
 
 		const hook2: LoadedHook = {
@@ -351,9 +342,6 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
-			setNewSessionHandler: () => {},
-			setBranchHandler: () => {},
-			setNavigateTreeHandler: () => {},
 		};
 
 		createSession([hook1, hook2]);